Chemical oxygen demand (COD for short) is the amount (expressed as oxygen equivalent) of a chemical compound—usually a strong oxidizing agent, for example potassium permanganate or potassium dichromate—which is consumed by oxidizable ingredients contained in a certain volume of liquid sample under the reaction conditions of a prescribed method. The COD value is an important parameter for classification of the degree of fouling in flowing water, and in wastewater, and clarification, plants, especially as regards organic impurities.
The fundamental principle of most methods for determining chemical oxygen demand is that a sample is treated with a known excess of an oxidizing agent, and consumption of the oxidizing agent is then ascertained, for example, through back titration of the remainder which is not consumed. The amount of the oxidizing agent consumed is converted into the equivalent amount of oxygen.
In the state of the art, several methods are known for automatedly determining the COD value of a liquid sample. German patent application DE 103 60 066 A1 describes an automated method for photometric determining of the COD value of a liquid sample, in the case of which a cuvette containing the liquid sample and potassium dichromate (as digestion agent) is, during a digestion time, heated under pressure-tight closure to a temperature above the atmospheric boiling temperature of the reaction mixture; wherein to accelerate the reaction time, the cuvette is subjected to a pressure of 5 to 10 bar, so that the reaction mixture can be heated to a temperature of e.g. 175° C., which lies clearly above the boiling temperature at atmospheric pressure. At the same time, the absorbance of the reaction mixture is ascertained at least one fixed wavelength in the cuvette during the total digestion. The change in absorbance serves as a measure for the concentration change of the oxidizing agent in the reaction mixture.
Chloride ions present in the reaction mixture can disturb the ascertaining of the chemical oxygen demand according to this method. For this reason, mercury(II)-sulfate (HgSO4) is added to the reaction mixture to mask the chloride-ions in the liquid sample. HgSO4 proves most effective when it is present in a quantity ten times that of chloride content. Mercury(II) salts are, however, highly poisonous, so a reaction mixture treated in such a manner cannot be returned directly to the water system. Instead, it must be disposed of in a complicated manner at high cost. Furthermore, the relatively high amounts of mercury(II)-salt which are required over the duration of the operation of the automatic analytical system pose a danger to operating personnel and to the environment. Consequently, ISO 6060, for example, allows a chloride content of only up to 1000 mg/1 in the sample.
A method is known from the article “Instrumentelle Bestimmung der organischen Stoffe in Wässern” (Instrumental Determining of Organic Materials in Water) in the Zeitschrift für Wasser-und Abwasser-Forschung (Journal of Water, and Wastewater, Research), Vol. 9, No. 1/76, pages 17 to 25, in which the chloride present in the liquid sample is not masked by mercury(II) salts, but instead separated from the liquid sample before the addition of an oxidizing agent. In this regard, before the addition of the oxidizing agent, the sample is so strongly acidified using pure, concentrated sulfuric acid at a proportion 1:1, that the chloride can be completely removed from the reaction mixture as hydrochloric acid, for example by gas purging. The described method functions continuously. In such a case, the sample is first brought together with the concentrated sulfuric acid and led through a degassing tube. In the degassing tube, air is blown through the sample, moving in the same direction as the sample and taking the separated hydrochloric acid with it. Half of the acidified and degassed sample is combined with the oxidizing reagent—a solution of potassium dichromate and silver sulfate in concentrated sulfuric acid at a proportion of 1:1—and led through a digesting unit, in which the organic materials present in the reaction mixture are oxidized. The digesting unit is composed of a long helical tube, which is heated by means of a thermostat. Half of the reaction mixture is then once again removed, and the excess potassium dichromate not consumed during the oxidation of the organic materials is ascertained. For this purpose, the remaining solution is combined with a standard solution of iron(II)- and iron(III)-sulfate, and led to a measuring unit via a mixing vessel. Via the measuring unit, the change in redox potential of the standard solution is measured, which comes about through oxidation of iron(II) ions into iron(III) ions by the potassium dichromate not consumed.
This method is quite complicated, and the corresponding automatic analytical apparatus is high in maintenance and susceptible to defects. For example, a series of liquid conducting lines between the reagent containers, the degassing tube, the digesting unit and the measuring unit are necessary. Such hose lines must regularly be changed, which requires a high maintenance effort. Added to this is the fact that, due to the complexity of the overall construction, it can easily happen that connections for the liquid transport lines are accidentally switched. Furthermore, the functional units of the analytical apparatus are complex and delicate in their construction. An example of this is the helical tube serving as the digesting unit; such a component is expensive to manufacture, relatively difficult to clean and susceptible to defects. The same is true for the degassing tube. This method and the corresponding apparatus are therefore rather poorly suited for use in process analytics.